1. Field of the Invention
The present invention pertains to a technique for measuring gas flow and/or volume in a pressure support system, and, more particularly, to a gas flow and/or volume measurement technique in which a pressure differential occurring between two points in a tortuous gas flow path in the pressure support system is used to measure the gas flow and/or gas volume passing through the tortuous gas flow path, thereby eliminating the need for a dedicated flow element in the gas flow path to create the pressure differential for flow/volume measurement purposes.
2. Description of the Related Art
Pressure support systems that provide a flow of gas to an airway of a patient at an elevated pressure via a patient circuit to treat a medical disorder are well known. For example, it is known to use a continuous positive airway pressure (CPAP) device to supply a constant positive pressure to the airway of a patient to treat obstructive sleep apnea (OSA) as well as other disorders. It is also known to provide a positive pressure therapy in which the pressure of gas delivered to the patient varies with the patient""s breathing cycle, or varies with the patient""s effort to increase the comfort to the patient, which is typically referred to as bi-level pressure support. It is further known to provide a positive pressure therapy in which the pressure provided to the patient changes based on the detected conditions of the patient, such as whether the patient is snoring or experiencing an apnea, hypopnea or upper airway resistance. This typically is referred to as an auto-titration mode of pressure support because the pressure support system automatically attempts to titrate the pressure support to suit the needs of the patient.
As shown in FIG. 1, a conventional pressure support system 10 typically includes a pressure generator 12, for example, a blower, piston, or bellows, that receives a supply of gas from a gas source, such as ambient atmosphere, as indicated by arrow A, and creates a flow of breathing gas, as indicated by arrows B, having a pressure greater than the ambient atmospheric pressure. A patient circuit 14, which is typically a flexible conduit, delivers the elevated pressure breathing gas to the airway of the patient. Typically, the patient circuit is a single limb conduit or lumen having one end coupled to the pressure generator and a patient interface device 16 coupled to the other end.
Patient interface device 16 connects patient circuit 14 with the airway of the patient so that the elevated pressure gas flow is delivered to the patient""s airway. Examples of patient interface devices include a nasal mask, nasal and oral mask, full face mask, nasal cannula, oral mouthpiece, tracheal tube, endotracheal tube, or hood. A single limb patient circuit shown in FIG. 1 includes an exhalation port 18, also referred to as an exhalation vent, exhaust port, or exhaust vent, to allow gas, such as expired gas from the patient, to exhaust to atmosphere, as indicated by arrow C. Generally, exhaust vent 18 is located in patient circuit 14 near patient interface device 16 or in the patient interface device itself.
More sophisticated pressure support devices include a flow sensor 20, pressure sensor 22 or both that monitor the flow and/or pressure of gas passing in patient circuit 14. The flow information can also be used to determine the volume of gas passing through patient circuit 14. The information from flow sensor 20 and/or pressure sensor 22 is used, for example, to control the pressure or flow of gas provided to the patient, monitor the condition of the patient, monitor the usage of the pressure support device (patient compliance), or any combination thereof. FIG. 1 illustrates a flow sensor 20 and pressure sensor 22 downstream of pressure generator 12.
As shown in FIG. 2, which illustrates an example of a typical flow sensor, flow sensor 20 includes a conduit 24 having ends 26 and 28 so that gas can flow through the conduit, as indicated by arrow D. A flow element 30 is provided in conduit 24 between ends 26 and 28 to create a pressure drop (xcex94P) in the conduit. That is, flow element 30 causes a pressure difference xcex94P between pressure P1 and pressure P2 so that xcex94P=P2xe2x88x92P1.
In one type of conventional sensor, pressure differential xcex94P is measured directly by a pressure sensor 32, which is connected to conduit 24 on each side of flow element 30 via ports 34 and 36. This pressure differential is used to calculate the flow of gas passing through conduit 24, which, in turn, is used to calculate the volume of gas flowing through conduit 24 over any given period of time.
In another type of conventional flow sensor, pressure differential xcex94P is not measured directly. Instead, a conduit is coupled between ports 34 and 36. The pressure differential between these ports causes a sidestream flow of gas to flow through this conduit connecting ports 34 and 36. A mass flow sensor 32xe2x80x2 is provided in place of pressure sensor 32 to measure the sidestream flow passing between ports 34 and 36. This sidestream flow is then used to calculate the flow of gas in conduit 24 and also the volume of gas flowing through conduit 24 over any given period of time.
The signal from flow sensor 20, whether from pressure sensor 32 or mass flow sensor 32xe2x80x2, is provided to a controller 38 where it is used for the purposes noted above, such as to control the pressure or flow of gas provided to the patient or monitor the patient""s usage of the medical device. One conventional pressure/flow control method involves providing a valve 40 in the patient circuit downstream of pressure generator 12 to exhaust a portion of the breathing gas output by the pressure generator through an exhaust conduit, as indicated by arrow E, thereby decreasing the pressure and flow delivered to the patient.
Another conventional pressure/flow control method involves controlling the operating speed of pressure generator 12, e.g., controlling the motor speed of a blower that is used to create a flow of gas so that the pressure generator outputs the gas at the desired rate or pressure without an additional pressure control valve. It is also known to use a combination of valve 40 and motor speed control to control the pressure or flow of breathing gas output to the patient.
Controller 38 receives the signals output from sensors 20 and 22 and controls the operation of valve 40, pressure generator 12, or a combination thereof in a feedback fashion based on these received signals. For example, in a simple CPAP device, controller 38 monitors the pressure or flow of breathing gas delivered to the patient and adjusts the pressure or flow in a feedback fashion to meet the desired prescription pressure level. In a more sophisticated bi-level pressure support system, where the pressure is greater during inspiration than during expiration, controller 38 receives the flow signal and the pressure signal from flow sensor 20 and pressure sensor 22, respectively, and uses this information to determine when the patient has transitioned from the inspiratory phase to the expiratory phase of the breathing cycle, or vice versa, to control the pressure accordingly. In the auto-titration mode of pressure support, where the flow of breathing gas and the pressure level thereof is controlled based on the conditions of the patient, these pressure and flow sensors, or other sensors, such as a microphone, are used to detect snoring, apneas, hypopneas, etc. The pressure and/or flow is then controlled to counteract or prevent these conditions.
An input/output device 42, such as a keypad, buttons, lights, LED or LCD display, and/or an audio device, is used to enter information and commands to the pressure support system and to display information. For example, an input device can be used to enter the operating pressure in a CPAP system, the inspiratory positive airway pressure (IPAP) and expiratory positive airway pressure (EPAP) in a bi-level system, and the maximum and minimum pressures and pressure change in an auto-titration system.
It can be appreciated that providing a conventional flow sensor in the pressure support system increases the complexity and cost of the device. This is so because the flow sensor must be manufactured and calibrated with a relatively high degree of precision to ensure an accurate output.
Others have attempted to avoid the use of a flow sensor altogether, for example, by monitoring the current or work performed by the pressure generator 12. As the patient breathes into patient circuit 14, the load on the pressure generator increases, this change in load can be detected and used to determine that the patient is breathing into the system. This monitoring technique, however, is relatively inaccurate, and, therefore, not suitable for quantitatively measuring the flow or volume of gas in the pressure support system, which is especially important in using the measured flow for triggering and cycling purposes.
It is, therefore, an object of the present invention to provide a pressure support system that overcomes the disadvantages associated with conventional pressure support systems. In particular, it is an object of the present invention to provide a pressure support system that allows for a relatively accurate flow or volume measurement of the gas passing therethrough, without the need for a dedicated flow element.
These objects are achieved, according to one embodiment of the present invention, by providing a pressure support system that includes a gas carrying conduit having a first end and a second end. Breathing gas from a gas source is received at the first end of the conduit. A pressure generator is provided at a first location along the conduit for generating a flow of breathing gas within the conduit. A first port is defined in the conduit at a second location, and a second port is defined in the conduit at a third location. The conduit is formed into a tortuous path between the second location and the third location so as to induce a pressure differential in the flow of breathing gas between the second location and the third location. In addition, a sensor associated with the first port and the second port measures a characteristic of the breathing gas in the conduit resulting from the pressure differential and outputs a signal indicative thereof. By making use of the pressure drop caused by the tortuous path, the need for a separate flow element found in conventional flow sensors is eliminated.
The present invention contemplates that the signal from the sensor can be used in the same manner as the signal from a conventional flow sensor. These uses include controlling the operation of the device, i.e., the pressure or flow delivered to the patient, and monitoring the usage of the device. The present invention further contemplates that the second and third locations, where the first and second ports are respectively provided, can be upstream or downstream of the pressure generator.
It is yet another object of the present invention to provide a method of providing pressure support that does not suffer from the disadvantages associated with conventional pressure support techniques and that can perform an accurate flow or volume measurement without a dedicated flow sensor. This object is achieved by providing a method that includes: (1) providing a gas carrying conduit having a first end adapted to receive breathing gas from a gas source and a second end, (2) generating a flow of breathing gas within the conduit via a pressure generator disposed at a first location along the conduit, and (3) providing a first port in the conduit at a second location and a second port in the conduit at a third location. The conduit is configured so as to define a tortuous path between the second location and the third location, thereby inducing a pressure differential in the flow of breathing gas between the second location and the third location. This process further includes measuring, via a sensor associated with the first port and the second port, a characteristic of the breathing gas in the conduit resulting from the pressure differential and outputting a signal indicative thereof. As noted above, this signal can be used for many purposes, and the first and second ports can be provided upstream or downstream of the pressure generating element.